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PIC18F2450/4450

DS39760A-page 70

Advance Information

2006 Microchip Technology Inc.

5.5

Program Memory and the

Extended Instruction Set

The operation of program memory is unaffected by the

use of the extended instruction set.

Enabling the extended instruction set adds eight

additional

two-word

commands

the

existing

PIC18 instruction set: ADDFSR, ADDULNK, CALLW,

MOVSF

, MOVSS, PUSHL, SUBFSR and SUBULNK. These

instructions

are

executed

described

Section 5.2.4 “Two-Word Instructions”.

5.6

Data Memory and the Extended

Instruction Set

Enabling the PIC18 extended instruction set (XINST

Configuration bit = 1) significantly changes certain

aspects

data

memory

and

its

addressing.

Specifically, the use of the Access Bank for many of the

core PIC18 instructions is different. This is due to the

introduction of a new addressing mode for the data

memory space. This mode also alters the behavior of

Indirect Addressing using FSR2 and its associated

operands.

What does not change is just as important. The size of

the data memory space is unchanged, as well as its

linear addressing. The SFR map remains the same.

Core PIC18 instructions can still operate in both Direct

and Indirect Addressing mode; inherent and literal

instructions do not change at all. Indirect Addressing

with FSR0 and FSR1 also remains unchanged.

5.6.1

INDEXED ADDRESSING WITH

LITERAL OFFSET

Enabling the PIC18 extended instruction set changes

the behavior of Indirect Addressing using the FSR2

proper conditions, instructions that use the Access

Bank – that is, most bit-oriented and byte-oriented

instructions – can invoke a form of Indexed Addressing

using an offset specified in the instruction. This special

addressing mode is known as Indexed Addressing with

Literal Offset or Indexed Literal Offset mode.

When

using

the

extended

instruction

set,

this

addressing mode requires the following:

The use of the Access Bank is forced (‘a’ = 0);

and

The file address argument is less than or equal

to 5Fh.

Under these conditions, the file address of the

instruction is not interpreted as the lower byte of an

address (used with the BSR in Direct Addressing), or

as an 8-bit address in the Access Bank. Instead, the

value is interpreted as an offset value to an Address

Pointer specified by FSR2. The offset and the contents

of FSR2 are added to obtain the target address of the

operation.

5.6.2

INSTRUCTIONS AFFECTED BY

INDEXED LITERAL OFFSET MODE

Any of the core PIC18 instructions that can use Direct

Addressing are potentially affected by the Indexed

Literal Offset Addressing mode. This includes all byte-

oriented and bit-oriented instructions, or almost one-half

of the standard PIC18 instruction set. Instructions that

only use Inherent or Literal Addressing modes are

unaffected.

Additionally, byte-oriented and bit-oriented instructions

are not affected if they use the Access Bank (Access

RAM bit is ‘1’) or include a file address of 60h or above.

Instructions meeting these criteria will continue to

execute as before. A comparison of the different

possible addressing modes when the extended

instruction set is enabled in shown in Figure 5-8.

Those who desire to use byte-oriented or bit-oriented

instructions in the Indexed Literal Offset mode should

note the changes to assembler syntax for this mode.

This is described in more detail in Section 19.2.1

“Extended Instruction Syntax”.

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